Abstract
Background/Aim: Although nutritional risk factors for developing complications in type 2 diabetes mellitus (T2DM) have been examined, the effect of protein intake on nephropathy is debated, and there is little research on retinopathy. This cross-sectional case-series study aimed to examine the risk factors, including nutritional status, for complications in patients newly diagnosed with T2DM. Patients and Methods: Fifty-four patients were recruited, based on the results of examinations of blood glucose and/or glycated hemoglobin level for T2DM. To evaluate nutritional status, blood and urine examinations were performed and the Food Frequency Questionnaire was administered. Two-way analysis of variance, Fisher’s exact test and logistic regression analyses were performed. Results: The patients were categorized into four groups: 24 without albuminuria and without retinopathy, four without albuminuria with retinopathy, 21 with albuminuria without retinopathy, and five with albuminuria with retinopathy. Logistic analysis of albuminuria revealed that estimated sodium intake was significantly independent as the explanatory factors of age, sex, and body mass index. Patients with retinopathy had significantly higher blood urea nitrogen, and significantly lower plasma total protein levels than patients without retinopathy, suggesting that retinopathy is related to a higher catabolic state. Through a questionnaire on food intake, patients with retinopathy had a significantly lower intake of fat and monounsaturated fatty acids and a significantly higher intake of iodine based on intake of seaweed, corrected for energy intake, than patients without retinopathy. Conclusion: The present study may lead to planning a large cohort study for examining nutritional risk factors related to complications in patients newly diagnosed with T2DM in Japan.
Patients with type 2 diabetes mellitus (T2DM) experience complications, such as nephropathy and retinopathy. Approximately 7-47% of patients with T2DM have microalbuminuria, which is found in nephropathy, while approximately 19-50% of patients have retinopathy in some races (1, 2). As these complications overlap or occur independently, their onset might be caused by common or independent factors (3-5). The main common factor is the diabetic condition, including disease duration. Other factors involved in the onset and development of complications have also been actively studied; however, to the best of our knowledge, nutritional conditions as risk factors have not been fully elucidated.
Due to the change in the Japanese lifestyle to the European style after the second world war, the rates of some lifestyle-related diseases, such as T2DM and hyperlipidemia, have increased (6). One of the effects of lifestyle changes is on the nutritional condition. Japanese people tend to consume more animal protein and lipids and lesser amounts of carbohydrate, plant protein and fiber than they used to (6). The change in nutritional habits is believed to be a cause of an increase in Japanese patients with T2DM. Since not all patients with T2DM experience complications, such as nephropathy or retinopathy, the factors that aggravate these complications were investigated. Whether the intake of a higher amount of protein in food is an aggravating factor for nephropathy is debatable (7, 8). While restricting the intake of protein in food to prevent nephropathy is still debated, increased intake of animal protein may adversely affect kidney function in patients with T2DM. However, to the best of our knowledge, no studies have described the effects of a high protein intake on eye function in patients with T2DM.
Previously, we reported that in an animal model of T2DM, db mice with a genetically deficient leptin receptor showed higher excretion of albumin in urine when fed a high protein and low carbohydrate diet than when fed a low protein and high carbohydrate diet (9-11). In addition, db mice fed the high protein low carbohydrate diet showed deterioration in glucose metabolism, and the genes involved in the renin–angiotensin system were modulated in the retina (12). These findings suggest that the high protein low carbohydrate diet may affect renal and retinal functions under diabetic conditions. In control mice, this diet led to increased urinary albumin, pathological findings in the glomeruli, urinary C-peptide excretion, and enhanced expression of renin–angiotensin system-related genes in the kidney (10). These findings encouraged us to examine the relationship between dietary conditions and the onset of T2DM complications in humans.
Studies on the complications of T2DM are being actively conducted worldwide; however, not so many studies have focused on the relationship between the diet and complications. This was a cross-sectional case-series pilot survey of newly diagnosed patients in Japan under drug-free conditions.
Patients and Methods
Patients. Patients at two hospitals (Uenomachi-Kajiya Clinic, a hospital specializing in DM, and Yotsueda Naika, a hospital specializing in renal diseases) were enrolled in the study between October 2018 and December 2021, as shown in Figure 1. The patients were referred to the hospital from other clinics or visited for health examinations and assessment of subjective symptoms. They were recruited without medication, with the qualitative urine examination showing proteinuria according to the test result (+/−, +, ++) (13) or an estimated glomerular filtration rate (eGFR) <60 ml/1.73 m2. After informed consent was obtained, 54 patients (35 men and 19 women) were enrolled.
Flowchart of the recruited patients. Alb: Albuminuria; eGFR: estimated glomerular filtration rate; Ret: retinopathy; –: without; +: with. The signs (+/−, +, ++) denote the result of the qualitative urine examination showing proteinuria (see Ref 13).
Examinations. The patients underwent general examinations, including physical measurements, blood chemistry analysis, and a quantified test for urinary albumin level to evaluate nephropathy. Additionally, a lifestyle questionnaire was administered. Based on the results of the quantified urinary test for albuminuria (13) and ophthalmological examinations evaluating retinopathy, conducted by specialist ophthalmological clinics in Kagoshima City, the patients were divided into four groups: Group 1: without albuminuria and without retinopathy; group 2: without albuminuria with retinopathy; group 3: with albuminuria without retinopathy; and group 4: with both albuminuria and retinopathy.
The estimated 24-h intake of sodium and potassium was calculated using the equation provided by Tanaka et al. (14, 15). The information on nutritional intake was collected using FFQg (Ver.5. 2016; KENPAKUSHA Corp., Tokyo, Japan) (16). From the collected data, the amount of energy consumed per day, after correction for energy intake or body weight (BW), was calculated (16, 17).
Statistical analysis. All continuous variables were calculated as the mean±standard deviation with the range (minimum and maximum). Categorical data are presented as integers. Statistical analyses were performed using a two-way analysis of variance (ANOVA) followed by a post hoc Dunnett’s test for continuous variables, and Fisher’s exact test for categorical variables. Statistical significance was set at p<0.05. Clinical parameters and nutritional measurements were selected as potential explanatory variables for albuminuria (objective variables) in the logistic regression analysis. SPSS software (Medical Model version 28, IBM, Tokyo, Japan), and EZR (version 1.63) were used for statistical analyses (18).
Ethics. Ethical approval was obtained from the Institutional Review Committee of Kagoshima University (approval number 180117 Eki). Informed consent was obtained from all participants before they completed the questionnaire. Written informed consent was obtained from all the enrolled patients.
Results
Characteristics of the enrolled patients. The physical and clinical characteristics of the patients are shown in Table I. The mean age was 52.3±14.8 years, and 35 out of the 54 patients were male. Glycated hemoglobin (HbA1c) was 9.2±2.5% (range=6.3-14.9%), which was above the threshold for diagnosis of T2DM according to the Japanese standard guidelines (19). Additionally, four patients with low HbA1c were diagnosed by fasting blood glucose (more than 126 mg/dl). There were 26 patients with albuminuria (48.1%) and nine with retinopathy (16.7%). There were 24 patients in group 1, four in group 2, 21 in group 3, and five in group 4. Thirty patients (55.6%) had at least one complication.
Characteristics of the enrolled patients (n=54).
Two-way ANOVA. In Table II, two-way ANOVA (two factors: albuminuria and retinopathy) revealed significant interactions between the two factors for, body mass index, systolic blood pressure, γ-glutamyltranspeptidase (GTP), creatinine, eGFR, high-density lipoprotein cholesterol, low-density lipoprotein cholesterol, hematocrit, and urinary albumin. Patients with more than one complication (group 4) had significantly higher systolic blood pressure, blood creatinine, and urinary albumin, and significantly lower eGFR in the post hoc test compared to patients without any complications. Patients in group 2 had significantly higher γ-GTP levels, compared to those in patients without any complications (group 1). In addition, patients in group 3 had significantly higher urinary albumin levels than those without any complications (group 1). Patients with albuminuria (groups 3 and 4) had significantly higher diastolic blood pressure, total cholesterol, urinary glucose, urinary urea nitrogen, urinary sodium, and estimated sodium intake, and significantly lower blood bilirubin than in patients without albuminuria (groups 1 and 2). Additionally, for the main effects, patients with retinopathy (groups 2 and 4) had significantly higher blood urea nitrogen, and significantly lower plasma total protein than in patients without retinopathy (groups 1 and 3).
Comparison of demographic and clinical variables between the four groups of patients: Group 1: without albuminuria and without retinopathy; group 2: without albuminuria with retinopathy; group 3: with albuminuria without retinopathy; and group 4: with both albuminuria and retinopathy.
Logistic analysis for the patients with or without albuminuria in the study cases. Significant differences in age, sex, and body mass index were observed between the patients with (groups 3 and 4) and without albuminuria (groups 1 and 2). Estimated sodium intake was independently evaluated in relation to these variables using logistic analyses (Table III).
Logistic regression analysis in patients with (groups 3 and 4) and without (groups 1 and 2) albuminuria.
The questionnaire on food intake. A questionnaire on food intake revealed the nutritional status of the patients. Through the questionnaire on food intake, patients with retinopathy (groups 2 and 4) had significantly lower intakes of fat, saturated fatty acids, monounsaturated fatty acids, zinc, vitamin B-1, and pantothenic acid, and a significantly higher iodine intake than patients without retinopathy (groups 1 and 3) (Table IV). The patients with retinopathy (groups 2 and 4) had a trend towards a lower intake of energy per day (p=0.076) compared to patients without retinopathy (groups 1 and 3) (Table IV), and this trend was eliminated (p=0.590) by correction for BW, suggesting that the lower intake of energy was explained by the lower BW in patients with retinopathy. Correcting the nutritional intake using an energy intake of 1,000 kcal, patients with retinopathy (groups 2 and 4) had significantly lower fat and monounsaturated fatty acids, and significantly higher iodine than in patients without retinopathy. In contrast, nutritional intake corrected for energy intake, such as sodium, appeared to have a significant interaction between albuminuria and retinopathy. Patients with albuminuria and retinopathy (group 4) showed significantly higher sodium intake corrected for energy intake than those without any complications (group 1), which was partly in accordance with the urine data shown in Table II.
Nutrient intake estimated by a questionnaire on food intake for the four groups of patients: Group 1: without albuminuria and without retinopathy; group 2: without albuminuria with retinopathy; group 3: with albuminuria without retinopathy; and group 4: with both albuminuria and retinopathy.
Discussion
From the perspective of nutritional factors involved in the development of T2DM complications, this case-series study showed that albuminuria can be related to higher estimated sodium intake, and retinopathy to increased catabolism, based on the lower plasma protein and higher blood urea nitrogen. In addition, patients with retinopathy (groups 2 and 4) showed deficient intake of several nutritional items but a higher intake of iodine based on intake of seaweed.
In the present study, among the patients (30 cases) with at least one of albuminuria or retinopathy, there were four patients with retinopathy only (13.3%) and 21 patients with albuminuria only (70.0%). The finding that not all patients with T2DM always had both complications suggests that internal and external factors are involved in the development of the respective complications (3-5). The recruitment procedure for the present study was based on kidney dysfunction; therefore, the number of patients with albuminuria may be higher than those with retinopathy. Actually, the number of patients diagnosed with albuminuria was high when newly diagnosed with T2DM in the study. To detect albuminuria in patients with T2DM, quantitative measurement of urinary protein should be considered during health examinations because there is a strong correlation between levels of urinary albumin and protein (20). In health check-ups, patients with higher blood glucose should always be examined for renal manifestations using quantitative measurement of urinary protein (13, 21).
There are many observational reports on the risk of developing T2DM complications, such as albuminuria and retinopathy (2, 22-24). Disease duration is a distinct risk factor. In addition, treatments, including drug therapy, may affect the development of complications. Therefore, the present study was designed to evaluate newly diagnosed patients without any treatment. However, the findings of the risk factors for albuminuria in this study, such as diastolic blood pressure, blood bilirubin, total cholesterol and urinary glucose, were mostly consistent with those reported in previous studies (3, 7, 22, 24). These findings suggest that the risk factors detected in the present study, including nutritional status, are reliable. In the present study, to evaluate nutritional conditions, objective values calculated from urine samples were used in addition to a questionnaire on nutrition.
Higher diastolic blood pressure, total cholesterol, urinary glucose, urinary urea nitrogen, and urinary sodium, and lower blood bilirubin are risk factors for albuminuria (Table II). These variables are consistent with published reports on patients with treatments (21, 25, 26). In addition, a higher estimated sodium intake was found to be a nutritional risk factor for albuminuria. These results are also consistent with those of previous reports (7, 8). Regarding factors with interaction, patients with both complications had higher systolic blood pressure, blood creatinine and urinary albumin, and lower eGFR than patients without complications. These findings from this observational study suggest that these should be considered as causal relationships.
Regarding retinopathy, lower plasma total protein, and higher blood urea nitrogen were identified as risk factors for diabetic retinopathy (Table II). Plasma total protein and blood urea nitrogen have been separately reported (27, 28). From a nutritional perspective, these findings may be related to increased catabolism, indicating a relative insulin deficiency (29). Under diabetic conditions, energy deficiency occurs at the cellular level. The presence of retinopathy only, and not albuminuria, indicates that different mechanisms are involved in the development of the two complications. In the interaction analysis, patients with retinopathy only had a higher γ-GTP than patients without either complication, suggesting that retinopathy is more sensitive to oxidative stress related to glutathione metabolism than is albuminuria (30, 31). Energy restriction is one of the nutritional applications for the general treatment of T2DM. However, the results of the present study should be interpreted with caution when energy restrictions are applied without examining eye conditions.
The questionnaire showed sodium intake was significantly higher in patients with both complications (group 4) than in the other groups, which is partly consistent with the urine analysis presented in Table II. This finding is consistent with previous reports (32, 33). In patients with retinopathy, although some items were detected as deficient factors, even after correction for energy intake, fat and monounsaturated fatty acids remained statistically significant, suggesting that fat and monounsaturated fatty acids should be consumed optimally to prevent retinopathy in patients with T2DM.
Epidemiologically, patients in Japan with T2DM have a higher prevalence of retinopathy than in other countries (34). In the present study, patients with retinopathy had a significantly higher intake of iodine (Table IV). In healthy individuals, an overdose of iodine leads to retinal disturbance due to oxidative stress, suggesting that a high intake of iodine induces retinopathy in patients with T2DM (35, 36). However, Chen et al. reported that no association between iodine intake and the development of retinopathy (37). Their report focused on iodized salt as the source of iodine, which is highly pure. In contrast, our study considered seaweed as the primary source of iodine. Namely, this discrepancy may imply that components other than iodine in seaweed might be involved in the onset of retinopathy. Japanese individuals reportedly have a high intake of iodine due to high consumption of seaweed (38). It is possible that the higher intake of iodine or other unknown substances in seaweed is related to the higher prevalence of retinopathy in patients with T2DM, in Japan. Further studies with larger cross-sectional and longitudinal perspectives are warranted.
The present study has limitations. Firstly, the number of patients (n=54) and clinics (n=2) were small. However, our results are consistent with those previously reported, particularly regarding the risk of albuminuria. In addition, by utilizing two-way ANOVA, we were able to partially compensate for the small sample size by evaluating the interaction and assessing the effects of renal complications and retinopathy separately in each group. Secondly, this was an observational study; therefore, the causal relationship between these results should be considered in future studies.
Conclusion
There are several observations regarding nutritional factors from the perspective of the development of T2DM complications: in patients newly diagnosed with diabetes, albuminuria might be related to a higher estimated sodium intake, and retinopathy might be related to a higher catabolic state as estimated by a lower level of plasma protein and higher level of blood urea nitrogen, compared to patients without retinopathy. In addition, patients with retinopathy had a deficient intake of several nutritional items but a higher intake of iodine based on intake of seaweed. These observations will be useful for planning a large cohort study to examine the risk factors, including nutritional status, in patients newly diagnosed with T2DM.
Acknowledgements
The Authors thank Editage (https://www.editage.jp) for editing the article. We also appreciate the efforts of the staff of the CKD preventive network in Kagoshima City, organized by the Kagoshima City Office.
Footnotes
Authors’ Contributions
EA and MH designed the study, and analyzed and drafted the article. YM and YN performed data acquisition. EA, MU and MH analyzed the data. KY, SK and YN performed data interpretation. EA and MH wrote the draft and edited it. All Authors critically revised the article and approved the final version.
Conflicts of Interest
The Authors declare no conflicts of interest.
Funding
This work was supported by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan [grant numbers 18K11038 to E. A.].
- Received May 16, 2024.
- Revision received July 5, 2024.
- Accepted July 10, 2024.
- Copyright © 2024 The Author(s). Published by the International Institute of Anticancer Research.
This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY-NC-ND) 4.0 international license (https://creativecommons.org/licenses/by-nc-nd/4.0).
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